Field
This disclosure relates to a non-aqueous electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same.
Description of the Related Art
A rechargeable lithium battery has recently drawn attention as a power source for a small portable electronic device. It uses an organic electrolyte solution and thereby, has twice or more high discharge voltage than that of a conventional battery using an alkali aqueous solution and as a result, has high energy density.
The organic electrolyte solution for a rechargeable lithium battery consists of a lithium salt such as LiPF6 and the like and an organic solvent. The organic solvent is required of low reactivity with lithium, minimized internal resistance for smoothly moving lithium ions, thermal stability within a vast temperature range, high compatibility with a negative active material, and a high dielectric constant enough to dissolve the large amount of the lithium salt. Examples of the organic solvent may mainly include cyclic carbonate such as propylene carbonate (PC), ethylene carbonate (EC), and the like; or linear carbonate such as dimethylcarbonate (DMC), diethyl carbonate (DEC), and the like and additionally, a hydrocarbon-based solvent such as 1,2-dimethoxyethane, diethoxyethane, and the like.
The PC among the organic solvents has a low melting point of about −49° C. and thus, excellent low temperature characteristic and also, has good compatibility with amorphous-based carbon and a high dielectric constant and thus, may dissolve the large amount of inorganic lithium salt. However, the PC has high viscosity and is inserted between carbon layers of a negative electrode during the charge when used with a crystalline carbon-based negative active material such as graphite and then, decomposed and thus, produces propylene gas and lithium carbonate, decreases capacity, and increases irreversible capacity. This irreversible capacity may be primarily caused by structural characteristic of carbon and vary depending on a degree that an electrolyte solution is reduced on the side where the carbon contacts with lithium and a degrees that a proactive layer of an electrolyte solution is formed on the surface of the carbon. On the other hand, the EC does not react with a graphite-based negative active material and may be easily applied to a battery using crystalline carbon as a negative electrode and also, has a high dielectric constant and thus, may dissolve the large amount of a lithium salt. However, the EC has high viscosity and a high melting point of about 36° C. and thus, may not secure a low temperature performance.
In addition, the linear carbonate such as DMC, DEC, and the like has low viscosity and is easily intercalated among negative active materials and may decrease irreversible capacity of the battery and also, has small reactivity with lithium but a low dielectric rate and thus, may not dissolve the large amount of a lithium salt. Particularly, the DMC has high electric conductivity and thus, may be applied to a battery with a high current and a high voltage but has a high melting point (about 4.6° C.) and thus, bad low temperature characteristic. In addition, the organic solvent such as dimethylformamide, acetonitrile, and the like has a high dielectric constant but high reactivity with lithium and accordingly, may not be substantially used.
Accordingly, a method of variously adding at least one organic solvent to an electrolyte, for example, adding the DEC having good low temperature characteristic to the EC/DEC has been suggested in order to complement drawback of each electrolyte solvent but still has a problem of insufficiently improving low temperature characteristic, having a low temperature of decomposing an active material, and hardly securing safety.